Concrete is a mixture of paste and aggregates, or rocks. The paste, composed of cement and water, coats the surface of the fine (small) and coarse (larger) aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete. Within this process lies the key to a remarkable trait of concrete: it's plastic and malleable when newly mixed, strong and durable when hardened. These qualities explain why one material, concrete, can build skyscrapers, bridges, sidewalks and superhighways, houses and dams.
Cement's chemistry comes to life in the presence of water. Cement and water form a paste that coats each particle of stone and sand—the aggregates. Through a chemical reaction called hydration, the cement paste hardens and gains strength. The quality of the paste determines the character of the concrete. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement. High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete, allowing it to be properly placed, consolidated, and cured. A properly designed mixture possesses the desired workability for the fresh concrete and the required durability and strength for the hardened concrete. Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent.
Curing begins after the exposed surfaces of the concrete have hardened sufficiently to resist marring. Curing ensures the continued hydration of the cement so that the concrete continues to gain strength. Concrete surfaces are cured by sprinkling with water fog, or by using moisture-retaining fabrics such as burlap or cotton mats. Other curing methods prevent evaporation of the water by sealing the surface with plastic or special sprays called curing compounds. Special techniques are used for curing concrete during extremely cold or hot weather to protect the concrete. The longer the concrete is kept moist, the stronger and more durable it will become. The rate of hardening depends upon the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions. Concrete continues to get stronger as it gets older. Most of the hydration and strength gain take place within the first month of concrete's life cycle, but hydration continues at a slower rate for many years. Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles and other components of the concrete to form a solid mass. The empirical formula of concrete can be written as:2Ca3SiO5+7H2O→3(CaO).2(SiO2).4(H2O)(gel)+3Ca(OH)2 where the exact ratios of the CaO, SiO2 and H2O in C—S—H can vary.
Many types of concrete are available, distinguished by the proportions of the main ingredients used. In this way or by substitution for the cementitious and aggregate phases, the finished product can be tailored to its application. Strength, density, as well chemical and thermal resistance are variables. Concrete production is the process of mixing together the various ingredients—water, aggregate, cement, and any additives—to produce concrete. Concrete production is time-sensitive. Once the ingredients are mixed, workers must put the concrete in place before it hardens.
Concrete has a relatively high compressive strength and a lower tensile strength. To void this weakness, it is usually reinforced with materials that are strong in tension. The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal expansion and shrinks as it matures. All concrete structures crack to some extent, due to shrinkage and tension. Concrete that is subjected to long-duration forces is prone to creep. Different mixes of concrete ingredients produce different strengths. Concrete can be damaged by many processes, such as the expansion of corrosion products of the steel reinforcement bars, freezing of trapped water, fire or radiant heat, aggregate expansion, sea water effects, bacterial corrosion, leaching, erosion by fast-flowing water, physical damage and chemical damage (from carbonatation, chlorides, sulfates and distillate water).
Concrete recycling is an increasingly common method for disposing of concrete structures. Concrete debris was once routinely shipped to landfills for disposal, but recycling is increasing due to improved environmental awareness, governmental laws and economic benefits. Concrete, which must be free of trash, wood, paper and other such materials, is collected from demolition sites and put through a crushing machine, often along with bricks and rocks. Reinforced concrete contains rebar and other metallic reinforcements, which are removed with magnets and recycled elsewhere. Crushed recycled concrete can sometimes be used as the dry aggregate for brand new concrete if it is free of contaminants, though the use of recycled concrete limits strength and is not allowed in many jurisdictions. On 3 Mar. 1983, a US government-funded research team estimated that almost 17% of worldwide landfill was by-products of concrete based waste.
In 1923, Bronsted and Lewis, separately, introduced the modern acid nomenclature. The first saw them as molecules capable of delivering an Hydrogen ion to other molecules that could accept it, and drew attention to the fact that the dimensionally tiny hydrogen ion could create a giant electric field in proportion, hence a very high polarization of the space around it. Lewis revolutionized the concept of acid by dislodging the concept from hydrogen ion and, indeed, from hydrogen itself. Due of this scientist, today we call acids compounds that do not even contain hydrogen ions: an example is BF3, Boro Trifluoride, which also disputes the common acid concept as a liquid in aqueous solution, being gaseous at a temperature above 12 degrees Celsius at atmospheric pressure. Such molecule categories are called Lewis Acids and have the feature of having an electronic duplex, in an outer orbital of the atom, not engaged in a chemical bond and behave in a particular way to the point that it is referred to as “Lone pair”. Lewis classified protonic acids as secondary acids by reserving the title of primary acids to those who are able to accept the pair of Lone Pairs electrons. Therefore, based on the Lewis classification, the hydrochloric acid HCl, the Sulfuric acid H2SO4, etc., are not primary acids due they are complex molecules of electrons and therefore cannot accept Lone Pairs.
Therefore, acid as a chemical molecule that if isolated, homogeneous and marketable, is not a waste but a value, finding on the market always a viable location and virtually immediate saleability. If, however, the acid is mixed with other substances, it is hardly recoverable with an economically viable process. At these conditions, it becomes a waste and therefore its disposal has an high cost, because it is necessary to treat it to make it harmless. The optimal disposal process for these acid waste is the one that does not just destroy the molecule, but let them react obtaining other products that are still valid on the market or even more valid and beneficial for human health and/or the environment than the one from which start the process.
An acid is a molecule or ion capable of donating a hydron (proton or hydrogen ion H+), or, alternatively, capable of forming a covalent bond with an electron pair (a Lewis acid). Acidity (or the amount of acid in a given substance) is measured with a number called pH, and acids have a pH less than 7. Chemically, acids are chemicals that contain positive hydrogen ions. The strength of an acid refers to its ability or tendency to lose a proton. A strong acid is one that completely dissociates in water; in other words, one mole of a strong acid HA dissolves in water yielding one mole of H+ and one mole of the conjugate base, A−, and none of the protonated acid HA. In contrast, a weak acid only partially dissociates and at equilibrium both the acid and the conjugate base are in solution. There are numerous uses for acids. In the chemical industry, acids react in neutralization reactions to produce salts. Acids are used as catalysts in industrial and organic chemistry.
Acid solutions treatment/neutralization is exothermic and may be potentially hazardous. It is important to dispose of acids with very low pH (<2) safely. If the acid doesn't have heavy metals or other toxic substances dissolved in it, neutralizing the pH to a less acidic level (pH 6.6-7.4) allows to dispose of the substance in the standard sewer system. If heavy metals are present, the solution must be treated as hazardous waste and disposed of through the proper channels. Some hazardous wastes were disposed of in regular landfills. This resulted in unfavorable amounts of hazardous materials seeping into the ground. These chemicals eventually entered to natural hydrologic systems. Many landfills require countermeasures against groundwater contamination.
Asbestos is a fibrous material which includes mineral silicates having a markedly fibrous asbestiform mineral growth habit and belonging to the amphibole or serpentine families. The mineral silicates are made up of incombustible, chemically-resistant, inert, phono-absorbing, flexible and tensile fibers. Asbestos mineral deposits can be found throughout the world and are still mined in Australia, Canada, South Africa and the former Soviet Union.
The chemical and physical properties of asbestos, namely its heat resistance, tensile strength and insulating properties, have rendered it one of the most important inorganic materials for industrial uses and technological applications. Asbestos minerals have been used in the construction of building materials such as cement products, acoustic and thermal sprays, pipe and boiler wraps, flooring and roofing materials, plasters, paints and many others.
Asbestos is characterized by a crystal formation of long, thin fibers, which makes asbestos quite different from other materials. Based upon its crystalline structure, asbestos may be classified as either serpentine or amphibole.
Serpentine asbestos has a sheet or layered structure. Serpentine minerals are usually associated with ultramafic Mg-rich rocks such as altered basalt which have been changed at different temperatures and in the presence of water in an alteration process known as serpentinization. The only member of the serpentine group, chrysotile, also known as “white asbestos”, is the most common type of asbestos found in buildings and is the predominant fibrous form of serpentine. Chrysotile is a fibrous mineral which does not burn or rot, is resistant to most chemicals, is flexible and possesses high tensile strength. This unique combination of properties makes chrysotile ideal as a major component of lightweight reinforced cement products, friction materials, high temperature seals, gaskets and a host of other materials. Chrysotile has been known for over 2000 years, being used initially for cremation cloths, oil lamp wicks and other textiles. In the 19th Century, chrysotile was first mined commercially in the Urals (Russia), Italy and Canada. Although it is the least abundant of the three traditional serpentine minerals, chrysotile accounts for some 95% of world asbestos production, and hence, is of key importance when the health effects of serpentine dust are considered. Chrysotile makes up approximately 90 to 95% of all asbestos used in buildings in the United States.
Amphibole asbestos, in turn, has a chain-like structure. There are five types of asbestos in the amphibole group: (1) amosite, which is the second most prevalent type of asbestos found in building materials and is also known as “brown asbestos”; (2) crocidolite, or “blue asbestos,” which is used in specialized high-temperature applications; (3) anthophyllite; (4) tremolite; and (5) actinolite. Anthophyllite, tremolite and actinolite are rare and are mainly present as contaminants in other minerals.
The amphibole group of minerals that include crystalline asbestos is based on the double-chain silicate tetrahedral structure which is cross-linked with bridging cations including magnesium, iron, calcium and sodium. The basic structural unit of amphiboles is (Si4O11)-6. The empirical formula of amphiboles can be written as:W0-1X2Y5Z8O22(OH,F)2 where W═Na+1 or K+1 in the A site with 10 to 12 fold coordination; X═Ca+2, Na+1, Mn+2, Fe+2, Mg+2, Fe+3, in an M4 site with 6 to 8 fold coordination; Y═Mn+2, Fe+2, Mg+2, Fe+3, Al+3 or Ti+4 in an Ml octahedral coordination site; and Z═Si+4 and Al+3 in the tetrahedral site.
The empirical formula of crocidolite (riebeckite asbestos) can be written as Na2 (Fe2+, Mg)3 Fe3+2 Si8 O22 (OH)2. Iron can be partially substituted with Mg2+ within the crocidolite structure. Crocidolite fiber bundles typically easily disperse into fibers that are shorter and thinner than other amphibole asbestos fibers which are similarly dispersed. However, crocidolite fibrils are generally not as small in diameter as chrysotile fibrils. In comparison with other amphiboles or chrysotile, crocidolite has a relatively poor resistance to heat. Its fibers, instead, are used extensively in applications requiring good resistance to acids. Crocidolite fibers have fair to good flexibility, fair spinnability, and a texture ranging from soft to harsh.
In amosite (grunerite asbestos), the Fe2+ to Mg2+ ratio varies, but is usually about 5.5:1.5. Amosite fibrils are generally larger than crocidolite fibrils, but are smaller than particles of anthophyllite asbestos similarly comminuted. Amosite fibrils typically have straight edges and characteristic right-angle fiber axis terminations.
Anthophyllite asbestos is a relatively rare, fibrous, orthorhombic, magnesium-iron amphibole, which occasionally occurs as a contaminant in talc deposits. Anthophyllite fibrils are typically more massive than those of other common forms of asbestos.
Finally, tremolite asbestos (a monoclinic calcium-magnesium amphibole) and actinolite asbestos (an iron-substituted derivative of tremolite asbestos) rarely occur in the asbestos habit and yet are common as contaminants of other asbestos deposits. Actinolite asbestos occurs as a contaminant fiber in amosite deposits, whereas tremolite asbestos occurs as a contaminant of both chrysotile and talc deposits. Tremolite asbestos fibrils range in size but may approach the dimensions of crocidolite and amosite fibrils.
The mechanisms of amphibole breakage are important biologically with regard to resultant particle number, surface area and general respirability (all of which control penetration to target cells and delivered dose), and also with regard to expressed chemical information contained on the fiber surface.
In nature, asbestos samples usually contain different cations from those used to describe the double-layer structure. Al+3 and Fe+3 may substitute for Si+4, and Fe+2, Fe+3, Mn+2 and Ni+2 can all substitute for Mg+2 to a greater or lesser degree. These substitutions may be summarized in a chemical formula written as:(Mg3-x-yRx+2Ry+3)(Si2-yRy+3)O5(OH)4 where R+2═Fe+2, Mn+2 or Ni+2 and R+3═Al+3 or Fe+3.
Normally, asbestos-containing materials (ACMs) in buildings do not pose a hazard to occupants and workers in those buildings except when asbestos fibers become airborne and are inhaled. In other words, intact, undisturbed asbestos-containing materials generally do not pose a health risk. It is when the asbestos-containing materials are damaged, disturbed or deteriorated over time that the asbestos-containing materials release asbestos fibers into the air inside the buildings and may become hazardous and pose an increased risk. As a toxic substance and known carcinogen, asbestos can cause several serious diseases in humans. Symptoms of these diseases typically develop over a period of years following asbestos exposure.
Because it has been unquestionably linked to lung cancer, asbestosis and pleural mesothelioma, asbestos is now considered to be a human health hazard. Furthermore, the use of asbestos is currently forbidden in several technologically-advanced countries.
Each country may set its own standards as to the definition of a hazardous or dangerous asbestos-containing material. For instance, the Environmental Protection Agency (EPA) in the U.S. classifies any material containing more than 1 wt % asbestos as an asbestos-containing material (ACM); a similar classification is defined in Italy by D. L. 277, 15 Aug. 1991.
Asbestos-containing materials can be basically divided into friable and compact asbestos materials. Friable asbestos designates any asbestos-containing material that can be easily crumbled or powdered when dry, and is normally composed of 70 to 95 wt % asbestos fibers. In general, friable asbestos in building materials can be found in artificial ashes and embers for gas-fired fireplaces, cavities, partitions of floors and ceilings, insulation of electrical wires, and insulation of panels, etc. In turn, compact asbestos designates an asbestos-containing material consisting in a composite material in which asbestos fibers are embedded in a cement or polymeric matrix. Compact asbestos is not prone to release fibers unless it is sawed or scratched by mechanical tools. In general, compact asbestos in building materials can be found in bonding and finishing cements, masonry fillers, mortars, mastics, asbestos-cement products having generally 4 to 15 wt % chrysotile asbestos and/or 0 to 6 wt % amphibole asbestos, etc.
A number of methods, some of which are patented, have been proposed to destroy or disrupt the molecular structure of asbestos and render asbestos less harmful to human health. The main processes used to transform asbestos into inert materials have been traditionally based on chemical treatment (typically by applying acid), thermal treatment and mechanochemical treatment. More recently, methods using a biochemical and microbiological approach have been developed.
Regarding acid-based chemical treatments, various methodologies that include the use of organic or mineral acids have been developed for the transformation of asbestos-containing materials and the rendering of secondary, recyclable and often reusable materials. In particular, mineral acids such as hydrofluoric acid, hydrochloric acid and sulfuric acid, as well as organic acids such as formic acid and oxalic acid, have been used to treat asbestos.
Regarding thermal treatments, it is well-known that dehydroxylation of chrysotile asbestos occurs in the range 650 to 750° C. It is also known that, at around 1000° C., the fibrous structure of chrysotile asbestos becomes loose and a new crystalline structure (forsterite and/or enstatite) is formed. Further, amphiboles such as crocidolite asbestos decompose at around 1050 to 1100° C. through a complex reaction path involving iron oxidation, which leads to formation of pyroxene, enstatite, hematite and cristobalite. Tremolite asbestos, in turn, decomposes into diopside, enstatite and cristobalite.
Vitrification is one of the most effective thermal processes for treatment of asbestos. The benefit of vitrification derives from the complete destruction of the fibrous structure and the formation of a glass-forming mixture which can be recycled as secondary glass material, due to the fact that asbestos-containing materials do not contain heavy metals. For instance, in the INERTAM-Europlasma process, vitrification of asbestos-containing materials is carried in a cylindrical furnace by a plasma torch at 1600° C. This is currently the only method of conversion of asbestos-containing materials that has been successfully adapted from a lab scale to a fixed, large-scale industrial plant. However, the process costs for rendering asbestos inert using this plasma-torch-based large-scale industrial process are, unfortunately, prohibitively high.
As to the biochemical and microbiological processes for treating asbestos, the best existing process known nowadays comprises the disintegration of the crystal planes of brucite (oxygen-magnesium) which are present within the crystalline planes of chrysotile as an indirect effect of metabolism of the bacterial cultures. Due to the presence of metabolites secreted by bacteria, decomposition of the crystal planes appears to be caused by acidification of the reaction environment. The process includes the steps of preparing an acid liquid/suspension by subjecting a food industry waste material to mixed bacterial and fungal growth and/or fermentation and treating the asbestos-containing materials with the acid solution/suspension obtained from the mixed fermentation at a temperature of 120 to 170° C. and a pressure of 2 to 10 bar. While this process may be effective, it requires from half a day to several days for completion, and is thus not sufficiently productive.
Regarding Hydroxyapatite (HAp) is a calcium phosphate chemically similar, in morphology and composition, to the mineral component of bones and hard tissues in mammals. It is one of few materials that are classed as bioactive, meaning that it will support bone ingrowth and osseointegration when used in orthopedic, dental and maxillofacial applications. The empirical formula of Hydroxyapatite can be written as:Ca5(PO4)3(OH)
The chemical nature of hydroxyapatite lends itself to substitution, meaning that it is not uncommon for non-stoichiometric hydroxyapatites to exist. The most common substitutions involve carbonate, fluoride and chloride substitutions for hydroxyl groups. Particularly, it has a hexagonal structure and a stoichiometric Ca/P ratio of 1.67, which is identical to bone apatite. An important characteristic of hydroxyapatite is its stability when compared to other calcium phosphates. Thermodynamically, hydroxyapatite is the most stable calcium phosphate compound under physiological conditions as temperature, pH and composition of the body fluids and it decomposes at temperature of about 800-1200° C. depending on its stoichiometry.
Accordingly, there remains a need for a safe disposal method capable of acids neutralization and destroy concrete and/or asbestos-containing materials (ACMs) that is not only applicable in large-scale industry, but also productive and non-hazardous to human health and the environment.